Microminiaturization of optical devices has been realized by using silicon (Si) optical wire waveguides having large differences in refractive index between the cores and the surrounding areas. A typical section size of a Si optical wire waveguide in a 1.55-μm wavelength band is 220 nm×450 nm. Because of strong optical confinement due to a large refractive-index difference, the radiation loss can be restricted to a small value even in a curved waveguide having a curvature radius of 5 μm or smaller. By applying the highly-advanced CMOS process technique, optical integrated circuits each including a number of optical or electronic micro devices can be mass-produced. Accordingly, optical devices are expected to be used not only in optical interconnects between devices and boards but also in large-capacity optical interconnects between/in chips that utilize the WDM (wavelength division multiplexing) technique.
To be used in optical interconnects, optical devices need to have the functions to transmit and receive optical signals. To be applied to optical interconnects between/in chips, optical devices should be made smaller in size, have lower power consumptions (higher efficiencies), and operate at higher speeds.
As for the receiving side, an efficiency in the neighborhood of 1 mA/mW and a band range of several GHz to several tens of GHz have been realized by Ge photodetectors of a waveguide type that are integrated with a Si optical wire waveguide. Such Ge photodetectors are 5 to 10 μm in length and are several μm in width.
Therefore, there is a need for Si optical modulators that are small in size, have low power consumptions (high efficiencies), have low insertion losses, operate at high speeds, and achieve sufficient extinction ratios. To be driven with a CMOS circuit, such optical modulators should preferably have lower drive voltages. However, there is a trade-off relationship between those requirements, and it is difficult to satisfy all the requirements at once.